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September 16, 2021 03:33 pm GMT

How to create a Smart Contract to mint an NFT

In this article we are going to create a smart contract to mint an NFT on Ethereum.

Setup the project

To start the project we will use the hardhat that will provide us with a boilerplate to deploy the contract, do tests, etc.

Installing the hardhat

We will install hardhat as development dependency using the yarn.

Open a new terminal and run these commands:

mkdir nft-tokencd nft-tokenyarn init -yyarn add hardhat -D 

In the same directory where you installed Hardhat run:

npx hardhat

Select Create an empty hardhat.config.js with your keyboard and hit enter.

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Creating the files and directories

Let's start by creating a directory called contracts in the root folder.

/node_modules/contractshardhat.config.jspackage.jsonyarn.lock

After creating the folder we can create our first file of our contract inside the directory, let's call it: factoryNFT.sol.

/node_modules/contracts | - factoryNFT.solhardhat.config.jspackage.jsonyarn.lock

Writing the smart contract

Now we go to the most anticipated part of the article, writing our contract.

Note: You can review the basics in my other article: How to create a smart contract to receive donations using Solidity

Let's start defining the pragma version:

factoryNFT.sol

pragma solidity ^0.8.3;

To help us, we will install the openzeppelin contracts package.

yarn add @openzeppelin/contracts  @openzeppelin/hardhat-upgrades

Now we can import it into the contract.

factoryNFT.sol

pragma solidity ^0.8.3;import "@openzeppelin/contracts/utils/Counters.sol";import "@openzeppelin/contracts/token/ERC721/extensions/ERC721URIStorage.sol";import "@openzeppelin/contracts/token/ERC721/ERC721.sol";

We made 3 imports:

  • Counters: is a useful one that will help us increment a tokenId for each new token.
  • ERC721URIStorage: we will provide some functions to handle our tokenURI, which contains metadata and image.

  • ERC721: will we provide some basic functions of the ERC721;

Inheriting the OpenZeppelin contract

Now we can start by defining the name of our contract and inheriting the OpenZeppelin contracts.

factoryNFT.sol

pragma solidity ^0.8.3;import "@openzeppelin/contracts/utils/Counters.sol";import "@openzeppelin/contracts/token/ERC721/extensions/ERC721URIStorage.sol";import "@openzeppelin/contracts/token/ERC721/ERC721.sol";contract FactoryNFT is ERC721URIStorage {     using Counters for Counters.Counter;     Counters.Counter private _tokenIds;    constructor() ERC721("Factory NFT", "FTN") {    }}

Creating our Mint function

Let's start by creating our function calling createToken, where it will receive as a parameter, our tokenURI.

factoryNFT.sol

pragma solidity ^0.8.3;import "@openzeppelin/contracts/utils/Counters.sol";import "@openzeppelin/contracts/token/ERC721/extensions/ERC721URIStorage.sol";import "@openzeppelin/contracts/token/ERC721/ERC721.sol";contract FactoryNFT is ERC721URIStorage {     using Counters for Counters.Counter;     Counters.Counter private _tokenIds;    constructor() ERC721("Factory NFT", "FTN") {    }    function createToken(string memory tokenURI) public returns (uint) {    }}

Now let's increment the amount of token in storage and save it in a variable.

factoryNFT.sol

pragma solidity ^0.8.3;import "@openzeppelin/contracts/utils/Counters.sol";import "@openzeppelin/contracts/token/ERC721/extensions/ERC721URIStorage.sol";import "@openzeppelin/contracts/token/ERC721/ERC721.sol";contract FactoryNFT is ERC721URIStorage {     using Counters for Counters.Counter;     Counters.Counter private _tokenIds;    constructor() ERC721("Factory NFT", "FTN") {    }    function createToken(string memory tokenURI) public returns (uint) {        _tokenIds.increment();        uint256 newItemId = _tokenIds.current();    }}

Now we can put the main function of the contract which is to mint our NFT and set its tokenURI.

factoryNFT.sol

pragma solidity ^0.8.3;import "@openzeppelin/contracts/utils/Counters.sol";import "@openzeppelin/contracts/token/ERC721/extensions/ERC721URIStorage.sol";import "@openzeppelin/contracts/token/ERC721/ERC721.sol";contract FactoryNFT is ERC721URIStorage {     using Counters for Counters.Counter;     Counters.Counter private _tokenIds;    constructor() ERC721("Factory NFT", "FTN") {    }    function createToken(string memory tokenURI) public returns (uint) {        _tokenIds.increment();        uint256 newItemId = _tokenIds.current();        _mint(msg.sender, newItemId);        _setTokenURI(newItemId, tokenURI);        return newItemId;    }}

Compiling and testing the Contract

Now to test the contract, we are going to compile it and write a test for it.

First of all let's go to the hardhat.config.js file and change the solidity version to ^0.8.3.

hardhat.config.js

/** * @type import('hardhat/config').HardhatUserConfig */module.exports = {  solidity: "^0.8.3",};

Plugins

Now to test our contract we need to install some plugins for hardhat to help us interact with Ethereum and test our contracts.

yarn add -D @nomiclabs/hardhat-ethers ethers @nomiclabs/hardhat-waffle ethereum-waffle chai

After adding the plugins we need to add a line in hardhat.config.js to setup the plugins.

hardhat.config.js

require("@nomiclabs/hardhat-waffle");/** * @type import('hardhat/config').HardhatUserConfig */module.exports = {  solidity: "^0.8.3",};

Writing the test

Now create a directory called test in the root folder and add called: factoryNFT.js.

/node_modules/contracts/test |- factoryNFT.jshardhat.config.jspackage.jsonyarn.lock

Now I am going to put the test below and explain each line to understand it.

const { expect } = require("chai");describe("Minting the token and returning it", function () {  it("should the contract be able to mint a function and return it", async function () {    const metadata = "https://opensea-creatures-api.herokuapp.com/api/creature/1" //Random metadata url    const FactoryContract = await ethers.getContractFactory("FactoryNFT"); // Getting the contract    const factoryContract = await FactoryContract.deploy(); //Deploying the Contract    const transaction = await factoryContract.createToken(metadata); // Minting the token    const tx = await transaction.wait() // Waiting for the token to be minted    const event = tx.events[0];    const value = event.args[2];    const tokenId = value.toNumber(); // Getting the tokenID    const tokenURI = await factoryContract.tokenURI(tokenId) // Using the tokenURI from ERC721 to retrieve de metadata    expect(tokenURI).to.be.equal(metadata); // Comparing and testing  });});

Now just compile and run the test and we will have the result!.

npx hardhat compile
npx hardhat test

So we come to the end, soon we will have a tutorial on how to get up to rinkeby testnet.

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Original Link: https://dev.to/emanuelferreira/how-to-create-a-smart-contract-to-mint-a-nft-2bbn

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